cRIO 9478 Weirdness

[Jeremy.Stevens]

I will try to keep this as simple as I can, but please be patient. I have a system with 4 identical cRIOs. Each has the following configuration: Slot 1 - AI Slot 2 - DI Slot 3 - DO 9478 Slot 4 - Empty Slot 5 - AI Slot 6 - Di Slot 7 - DO 9478 All are connected to terminal blocks which distribute the signals to various devices around the 2 skids. The solenoids devices connected to the DO card in the 3rd slot of the 4th chassis energize as soon as power is applied to the system. This happens as long as the cable between the terminal blocks and the DO card is connected to both even if the card is not in the chassis. It happens on all channels. It doesn't matter if the cRIO is powered on or not. When the cable is disconnected, resistance check between all wires shows no short. When card is swapped with another from a different chassis, the problem does not follow the card. The card and wiring are done for the proper sinking configuration. It is acting as though there is contact with ground on all channels of the card between the device and the card, but all resistance checks between the device and the module show isolation. If it were one or, perhaps, two channels, I would suspect the cable, but it's on all channels. I'm probably missing something stupid, but if there are any suggestions I would be very grateful.

[Jeremy.Stevens]

Just as an FYI, if the company that built your panel wires your 9478 such that one of the VSUP wires is connected to DCCOM even after they said they tested and verified all the wiring. This is something that will allow current flow through all channels of the card.

[MarkCG]

Definitely sounds like a wiring issue, given that it occurs with the card not plugged into the chassis. You also said it does not follow the module when you swap it with one in another chassis, so that rules out a bad module.

What I would do in this situation is first look at the module wiring and the schematic if you have one and try to find obvious mistakes. Those D-sub modules can be a bit tricky with what terminals do what, it's easy to make a mistake. Could be that the schematic is wrong too, so cross check with the NI 9478 getting started manual.

It could also be that the issue is downstream of the module wiring. Are the digital outputs wired to DIN rail terminal blocks in the control panel before going out to the solenoids? It could be a short circuit there.

If that fails, you may just have to rewire the module yourself. Unwire everything in the module terminal block, and rewire one channel at a time, test it, and keep doing that for each channel. Eventually you will find the problem. could be mislabeled wire. It helps do do this with an assistant so you can both check the signal path with multimeters and do continuity checks on each wire.

If I may get on my soapbox here for a minute though, this kind of problem ALWAYS happens when using this kind of PLC in enclosure+ wires running to device. It's just human nature that will always result in a few errors out when dealing with hundreds to thousands of wires. But I think it's rapidly becoming obsolete. If you can use IP67 distributed I/O available from many vendors now, that connect to your devices with commercial off the shelf cables, you are now eliminating the need to do run discrete wires to individual I/O module terminals.